How to modify accurate 3D models to protect technical know-how
Setup your decimate modifier on one of the objects.

Then, in object mode, select all objects (A) or all needed object (Shift+RMB).  

Make sure your decimated object is active (different color, usually brighter orange), if it's not, Shift+RMB on it.  

Use Ctrl+L to open the link menu and select "modifiers"


All the selected objects will now have the exact same modifiers as the active object.


About export : modifiers are applied when exporting, so it will be destructive. There can be a checkbox (depending on file format) to apply modifiers or not.